In blood plasma cholesterol exists and is transported in the form of plasma lipoprotein, that is, nonpolar lipids including cholesterol in blood plasma are combined with hydrophilic lipoprotein, which is water-soluble and thus facilitate their transport. Plasma lipoproteins can be divided into four classes: high-density lipoprotein (HDL), low-density lipoprotein (LDL), very-low-density lipoprotein (VLDL) and chylomicron (CM), wherein the physiological role of LDL is to deliver cholesterol to peripheral tissues, while HDL is to clear the cholesterol in the arterial wall and to deliver it back to the liver. Epidemiological and clinical studies have demonstrated a positive correlation between low-density lipoprotein cholesterol (LDL-C) concentrations and the incidence of arteriosclerotic disease such as coronary heart disease (CHD), while high-density lipoprotein cholesterol (HDL-C) has been demonstrated a negative correlation, that is, it has anti-atherogenic activities. Therefore, LDL-C and HDL-C are the most valuable risk factor index for cardiovascular and cerebrovascular diseases in clinical laboratory measurements of plasma or serum lipids.
Various methods have been used to measure cholesterol in HDL and LDL, including ultracentrifugation, chromatographic and electrophoretic techniques, and precipitation methods. Ultracentrifugation, in which method LDL or HDL are separated according to their density in a ultracentrifuge and then the cholesterol in them is measured, has been the basis for quantitative measurement in most studies and clinical practice; the electrophoretic methods developed later separate lipoproteins on a supported media such as cellulose acetate or agarose gel and then the cholesterol contents are measured by an enzymatic method; in precipitation methods, precipitation reagents coagulate lipoproteins other than HDL, the coagulated lipoproteins are separated by centrifugation and the HDL-C contents are determined by enzymatical measurement of the cholesterol in the supernate; in the first generation of precipitation methods for measurement of LDL-C, Polyvinyl Sulfate (PVS) and polyethylene glycol methyl ether (PEGME) are used to precipitate LDL, the precipitates are collected by centrifugation, and then the cholesterol contents are determined by enzyme reactions. All the methods mentioned above need special instruments, including ultracentrifuge, electrophoresis apparatus and centrifuge etc., and the procedures are complicated, hard to be automated, and thus can not be performed in the clinical laboratory.